1. Field of the Invention
This invention relates to a method and apparatus for denitrification of the effluent from an activated sludge sewage treatment system containing nitrogen compounds in the form of nitrates and nitrites by passage through a columnar filter, the filter media of which contain bacteria which convert the nitrogen compounds to nitrogen gas, with simultaneous removal of suspended solids. More particularly, the method and apparatus of this invention relate to periodic backwash of the filter with retention of a portion of the denitrifying bacteria which are then returned to the filter media to inoculate or "reseed" the filter for continued use.
2. Description of the Prior Art
Recent requirements in pollution control have placed limitations on the discharge of nitrogen compounds in treated sewage effluent into streams, and removal of such nitrogen compounds has thus received considerable attention.
Removal of nitrogen from waste water by the use of denitrifying bacteria involves, first, conversion of organic nitrogen and ammonia into nitrates, followed by removal of the oxygen by denitrifying bacteria to yield nitrogen gas. In each stage, the intermediate compounds are nitrites.
In the conversion of organic nitrogen and ammonia to nitrates adequate aeration must be provided, and this aerobic process also results in removal of carbon. However, carbon must be present during the denitrification by dentrifying bacteria. Accordingly carbon has to be reintroduced into the system, and this is commonly done by addition of methanol. The biochemical reaction which occurs when methanol is used as the carbon source results in production of nitrogen gas, carbon dioxide and water. The amount of methanol required is about three times the weight of nitrogen compounds to be removed. As is known in the art, other carbon sources can be used.
U.S. Pat. No. 3,709,364, issued Jan. 9, 1973, to E. S. Savage, discloses a method and apparatus for denitrification and clarification of nitrate-containing waste water, particularly adapted for sewage treatment. In this patent a filter bed is provided having a depth in excess of three feet with filter media comprising hard, rounded, gravel-like quartz sand of a size between 1-6 mm diameter. Denitrifying bacteria are supplied to the filter media.
Means are provided for periodic backwashing of the filter in the Savage system, whereby clean water is forced into the bottom portion of the filter, passed upwardly through a reticulated bottom, through the filter media and discharged to a backwash well. The backwash water is then returned to the aeration zone. In order to prevent removal of the entire denitrifying bacteria colony from the filter media during backwash, the patentee proposes several alternatives, one being diversion of a portion of the backwash dirty water from the backwash well to the influent line to the filter. Alternatively backwashing may be carried out by shortening the backwash cycle to the extent that a portion of the solids in the filter bed are retained, or a major portion of the filter bed may be backwashed while a minor portion at the bottom thereof is subjected to only slight backwashing. It will of course be recognized that complete backwashing is necessary in order to remove collected solids from the filter and to liberate trapped bubbles of nitrogen gas which accumulate in the filter bed.
The procedure of pumping backwash dirty water to the backwash well and then pumping a portion of the backwash dirty water back to the influent to the filter is undesirable in that additional pumps, connecting conduits, valving, controls therefor are required, and in that the concentration of bacteria in the backwash well is very low, thus requiring the return of a large volume of backwash dirty water in order to inoculate the filter bed with a sufficient proportion of the bacteria colony. The alternatives proposed by the patentee, such as shortening the backwash cycle or backwashing only the upper portion of the filter bed, obviously result in incomplete removal of collected solids and trapped bubbles of nitrogen gas, thus decreasing the efficiency of the filter bed when it is returned to use.